THE POINTING OF THE IRAM 30-M TELESCOPE

The original pointing model of the IRAM 30-m millimeter wavelength rad io telescope is based on nine parameters which are updated from specia l pointing measurements made approximately every two weeks. The pointi ng models derived from these measurements provide a pointing accuracy of similar to 3.5'' (rms) which, however, degrades slowly with time. A pplication by the observer of instantaneous pointing corrections recov ers the inherently precise operation of the telescope. The data analyz ed here show that the number of pointing parameters used for the 30-m telescope can be reduced to six because the astronomical positions of pointing sources are today accurately known, and because the telescope does not show hysteresis or long-term irreversible changes of its mec hanical behaviour so that the gravitational bending can be described b y a unique and time-independent, function, as should be the case. Ther e are indications that the inclination of the azimuth axis is influenc ed by seasonal thermal effects, which merit further monitoring for eve ntual quasi real time correction. The other pointing parameters show r andom variations in time which makes it difficult to trace any particu lar source which sets their ultimate accuracy, although we suspect res idual thermal effects to be the main source of perturbation.